Process of forming an arsenic sulfide mask

ABSTRACT

A process for forming masks employed in the manufacture of devices by photofabrication wherein arsenic sulfide is employed as one of the masking or lifting materials. Arsenic sulfide is soluble in solutions such as ammonia water while other masking or lifing materials which may be employed in such processes are soluble in acids or organic solutions. The arsenic sulfide when melted forms a razor sharp edge that can be superior to that of the original master mask.

United States Patent William 1. Lehrer Los Altos, Calif.

Apr. 18, 1968 Nov. 23, 1971 Fairchild Camera and Instrument CorporationSyosset, Long Island, N.Y.

Inventor Appl. No. Filed Patented Assignee PROCESS OF FORMING AN ARSENlCSULFlDE MASK 9 Claims, 7 Drawing Figs.

US. Cl ll7/5.5, 96/383, 117/8, 117/8.5, 117/37 R, 117/38, 117/124A,117/1243 Int. Cl B44d l/52, B44d 1/02 Field olSearch 117/38,

212,106, 5.5, 8.5, 8,106, 37 R, 124 A, 124 B; 96/36, 36.2, 38.3, 38.4;156/17 [56] References Cited UNITED STATES PATENTS 2,923,624 2/1960Hensler 1 17/5.5 X 2,995,461 8/1961 Boicey et a1. 1 17/5.5 3,115,42312/1963 Ashworth 1 17/5.5 X 3,376,139 2/1968 Giangualano et al. 96/3623,423,237 1/1969 Hutchinson 1 17/106 X OTHER REFERENCES M. C. Sneed andR. C. Brasted Comprehensive inorganic Chemistry Vol. 5, D. Van NorstranoCo., N.Y., 1956, p. 151

Primary Examiner-Ralph S. Kendall Assistant ExaminerC. K. WeiffenbachAltorney- Roger S. Borovoy PROCESS OF FORMING AN ARSENIC SULFIDE MASKBACKGROUND OF THE INVENTION This invention relates to a process forforming devices by photofabrication and to a process for forming masksemployed in such processes.

In many arts, such as the electronic art and semiconductor art, a filmor layer of material is fonned into a predetermined configuration on aselected surface. For example, after forming a monolithic circuit, it isnecessary to make electrical contact with the numerous elements orregions of the device. This may be accomplished by forming apredetermined configuration of contacting material over the surface of amonolithic device. In other instances, it is desired that apredetermined configuration in the form of a narrow cut in a passivatinglayer of material (e.g., silicon monoxide) be made incident to forming adevice in or on a substrate. In other instances, independent masks (thatis, masks not on the surface of an actual device) are formed foremployment in exposing and processing of an actual device. All of thesevarious applications may involve the formation of a film or layer ofmaterial into a predetermined configuration.

Predetermined configurations of a film of material have been formed byetching, by lifting, or by a combination of these techniques. Etchinginvolves the selective chemical dissolution of undesired regions of thefilm material from the surface. Certain materials are impractical orvery difficult to etch, (e.g., Ba Ti Nichrome). In addition, etching,being a diffusion limited process, always undercuts to some degree thefilm, thereby reducing the accuracy of the process. For example, a onemicron diameter opening in a deposited oxide film, or a two by twomicron aluminum square, cannot be readily formed by etching. Also, thereliability of the etching process in part depends upon the adherence ofa photoresist layer to the film material being etched. This adherencecannot be readily checked so that the application of the etchant to thephotoresist can be a hit and miss operation.

The lifting process on the other hand involves depositing the film ofmaterial partially upon a surface and partially upon a lifting materialwhich is formed in a pattern on the surface. The lifting material isthen removed along with the film material deposited thereover leaving apattern of the film material on the surface desired. Organic liftingmaterials and metal film lifting materials are commonly used. Theorganic materials have limited temperature application since theygenerally decompose at elevated temperatures, that is, temperaturesgreater than 300 C. Organic lifting materials also have the shortcomingof being limited in the thickness of the layer that may be lifted, thatis, thickness in excess of several thousand angstroms are very difiicultto lift. Organic lifting materials also pose a contamination problemwhen employed in vacuum systems. The metal film lifting materialsusually have the shortcoming of microalloying or diffusing into thesubstrate at temperatures greater than 400 C., which alters the propertyof the film and in addition may alter the property of the semiconductorjunction, if employed in the particular device.

One recent significant improvement in the lifting processes involves theuse of halides, calcium flouride in particular, as a lifting material.This procedure is described in US. Pat. application Ser. No. 509,825,now abandoned, filed on an invention of William I. Lehrer, and assignedto the same assignee as this application. As described in thatapplication, Lehrers procedure comprises forming a pattern ofphotoresist material on a surface of a device, forming a layer of aGroup II (A) fluoride over the photoresist; removing the photoresistpattern thereby to leave a pattern on the device surface outlined byfluoride; depositing a metal film over both the fluoride and thepattern; and removing the fluoride to leave the metal film on thesurface of the device in the pattern of the photoresist material. In oneembodiment, Lehrer discloses the use of calcium fluoride as the Group II(A) fluoride lifting material. The use of materials such as calciumfluoride overcomes many of the disadvantages of previous metal andorganic lifting materials. For example, the metal halides, such ascalcium fluoride, decompose at high temperatures (e.g., temperatures inexcess of I,300 C.), and do not afiect the electrical and integratedcircuit arts, and they do adhere to common substrate materials. Mostimportantly with respect to the present invention, the halides may belifted by dilute acids and they are not affected by solutions commonlyemployed with photoresist images (alcohols, ketones, etc.

As effective as the halides are for lifting materials, they do have thedisadvantages that the lifting technique results in edge definition thatlimits the state of the art, and they are acid soluble; therefore anacid cleaning dip prior to further depositions cannot be performed. Theinvented process solves these problems and has all of the advantagesincident to the lifting process employing the halides and in addition,possesses many other advantages over the prior art processes.

SUMMARY OF THE INVENTION In a process for forming devices byphotofabrication, a layer of arsenic sulfide is formed on a surface andis formed into a pattern. The layer of arsenic sulfide may be melted.The formed pattern of arsenic sulfide may be used as a masking orlifting material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la illustrates a device orsubstrate surface;

FIG. lb illustrates a surface with a layer of material such as aphotoresist;

FIG. 10 illustrates the developed photoresist;

FIG. ld illustrates the developed photoresist with a layer of arsenicsulfide thereover;

FIG. 1e illustrates FIG. M with the photoresist material removed and thepattern of arsenic sulfide formed;

FIG. If illustrates the pattern of arsenic sulfide of FIG. 16 with anadditional film of material thereover;

FIG. lg illustrates the product of FIG. If with the arsenic sulfidematerial removed and has a film of material formed into a pattern;

Detailed Description of an Embodiment of the Invention At the outset, itshould be understood that the various steps as described hereinafter maybe arranged in a number of different sequences and various steps may beomitted consistent with employment of the invented process. For example,if the process employs a photoresist etching step, a halide materiallifting process, and the invented process, the three may be combined inat least nine (9) different ways and considering that all three are notabsolutely necessary, numerous other processes may be derived therefrom.In addition, while the invented process is described with reference toelectronic devices and semiconductor technology, in its broadest aspectsit has general application to the photofabrication of devices regardlessof the end use of the particular device. The term mask, as used herein,refers to masks actually formed on the device as a result of physical orchemical alteration of the device being formed as well as masks whichare independent of the device and employed to expose a photoresist orother similar material.

FIG. Ia shows a device or substrate 10 having a surface 12. The finallyformed device 10 may be a mask, a semiconductor device, such as anintegrated circuit, a metal part, a plastic part, or a glass surface orother similar surface. In accordance with the invention, a layer ofarsenic sulfide is to be formed into a pattern on surface 12. This maybe accomplished by etching or lifting material or a photoresist. Iflifting is selected, it is desirable to employ a lifting method such asshown in pending patent application Ser. No. 509,825, now abandonedinvented by William I. Lehrer, and assigned to the same assignee of thisinvention. In that patent application, a halide such as calcium fluorideis employed to perform the lifting process. It should be noted thatcalcium fluoride is soluble in dilute acids such as dilute nitric acid,dilute sulfuric acid or dilute hydrochloric acid. In the embodimentshown in FIG. lb, a layer of photoresist I4 is first formed on surface12. The photoresist material 14 may be a photoresist such as AZI350(autopositive) marketed by the Shipley Corporation. The photoresist isapplied in the form of a continuous layer by standard photoresistcoating apparatus. The photoresist is then exposed and developedaccording to well-known photoengraving techniques leaving an image 18 ofthe photoresist material such as is shown in FIG. 1c.

The surface 12 and photoresist image I8 are then covered with a layer ofarsenic sulfide (e.g., arsenic trisulfide, arsenic disulfide and/orarsenic pentasulfide). The arsenic sulfide material 20 shown in FIG. 1dmay be deposited by standard vacuum deposition techniques employingordinary vacuum deposition chambers having sources which operate attemperatures of about 700 C. The arsenic sulfide has excellentdeposition characteristics, particularly since it does not tend tospatter before the deposition process begins which results in theforming of a uniform regular layer of material. The formed layer ofarsenic sulfide is a relatively porous structure. In addition, it hasthe following characteristics and properties.

1. It does not affect the electrical characteristic of the semiconductorjunctions;

2. It does not alloy with metals commonly employed in the electrical andintegrated circuit arts;

3. It may be lifted or etched by basic solutions which do not affect theother components or materials associated with an integrated circuit orelectrical devices;

4. It adheres to common substrate materials such as silicon,

glass, ceramics, etc.;

. It is not affected by solutions employed to remove photoresist imagessuch as organic solutions;

6. It is not affected by solutions commonly employed to remove liftingmaterials such as the alkaline earth halides; and,

. It may be melted at 200 C. sealing pin holes and providing a verysharp edge resulting in excellent edge definition.

From the above properties, it should be noted that the photoresist,halides, and the arsenic sulfide films are etched or soluble indifferent solutions and consequently may be used together in a processwith the removal of one material not affecting the removal of the othermaterial. This provides a powerful versatility in processing.

Following the deposition of the arsenic sulfide material 20 (FIG. 1d thephotoresist image 18 is removed by submerging the substrate 10 orsubmerging the portion of substrate 10 having photoresist image 18 andarsenic sulfide material 20 thereon in a solvent such as acetone. Theapplication of the solvent removes the photoresist image 18 and theportion of the sulfide thereon leaving the mask or pattern 22 (FIG. 1eof arsenic sulfide material 20. This removal is possible notwithstandingthe fact that the photoresist image is covered with arsenic sulfide. Theporous structure of arsenic sulfide enables the solvent to penetrate itand thereby remove the undesired photoresist and the arsenic sulfidecovering it. The mask 22 while shown as a simple geometric form may takeany of the more complex forms commonly employed in the integratedcircuit masking art or in the photofabrication of devices.

In the above process, the photoresist material 14 (FIG. 1b) was placedon surface 12 prior to the deposition of the arsenic sulfide material20. It should be recognized that it is consistent and within the scopeof the invention to first deposit the arsenic sulfide material 20 andthen cover that surface with a layer of photoresist. The photoresistcould then be exposed and developed to uncover certain areas of thearsenic sulfide material 20. The uncovered arsenic sulfide materialwould then be removed by the application of a basic solution such asammonia water. Following the application of the basic solution, theremaining photoresist material would be dissolved by the application ofan appropriate organic solution such as acetone. In either instance,either the first described steps illustrated in FIGS. 1a-ld, or thesteps just described, the resulting structure would be substantially asshown in FIG. 1e, that is, a pattern of arsenic sulfide material onsurface 12.

The mask 22 (FIG. 12 may then be melted by heating to approximately 200C. This heating would in essence result in the formation of a glasslikesubstance having the desirable characteristics of a very sharp, smoothedge that provides excellent edge definition. The glass forms at arelatively low temperature and any semiconductor devices which mayreside in substrate 10 would be substantially unaffected. The meltingwould also seal any pinholes in mask 22 (FIG. 1e), thereby providing alayer of material substantially impervious to acids, such ashydrofluoric acid. This layer may be employed advantageously insemiconductor technology as an oxide etching mask or as any other mask.It may also be desirable to form another layer of photoresist over themask 22 and again etch the melted material by application of a basicsolution such as dilute NH.,OI-I.

The final step in this embodiment of the invented method is to use theformed arsenic sulfide mask 22 in the process of forming a device. Thismay involve the use of the mask 22 as an independent mask to exposephotoresist coated over another surface or as shown in FIGS. lf-lg,wherein the predetermined configuration of arsenic sulfide 20 isemployed to lift another material and thereby form this material. Asshown in FIG. If, a layer of material 26 is formed or deposited overarsenic sulfide 20. The arsenic sulfide (FIG. 1e), is then removed bylifting it with a basic solution such as ammonia water. Such a solutionreadily removes arsenic sulfide 20 and the overlying material 26,leaving a predetermined configuration 28 (FIG. lg) of film material 26intact. The predetermined configuration 28 has a form determined by mask22 (FIG. 1e).

In the embodiment shown in FIGS. la-Ig, the arsenic sulfide functions asa lifting mask or material. It is readily apparent that it could equallyfunction as an etching mask or a mask for the exposure ofa photoresistby merely altering the steps shown in FIG. 1 (d-g).

In another embodiment of the invention, the material employed to formthe arsenic sulfide in a predetermined configuration would be a halidelifting material such as calcium fluoride rather than the photoresist.The halide lifting material is first formed into a predeterminedconfiguration by the method shown in aforementioned patent applicationSer. No. 509,825, heretofore referred to. Following the forming of thearsenic sulfide by such a lifting procedure, the formed mask is employedto form an oxide etch. Subsequently, the arsenic sulfide may be formedinto different configurations by applying another layer of photoresistover the first formed pattern, exposing and developing the photoresistand removing the arsenic sulfide not covered by photoresist by theapplication of a basic solution, such as ammonia water. Thus, it ispossible by employing the instant invention to use alkaline earthhalides, photoresists, and arsenic sulfide in a combination of steps toform a device. This is possible since the photoresists, alkaline earthhalides, and the arsenic sulfides are all soluble in different classesof solutions. Thus, the use of arsenic sulfide as a masking material ora lifting material provides a powerful tool in the fabrication ofdevices and provides a new dimension in versatility in such fabrication.This advantage along with excellent edge definition and other heretoforelisted advantages are a few of the more important benefits which flowfrom this invention.

Although this invention has been disclosed and illustrated withreference to particular applications, the principles involved aresusceptible of numerous other applications which will be apparent topersons skilled in the art.

Iclaim:

I. In a process for forming devices by photofabrication, the stepscomprising;

forming a layer of arsenic sulfide on a surface; and

shaping said layer into a pattern by removing selected portions of saidlayer from said surface.

2. In a process for forming masks that are employed in the manufactureof devices by photofabrication the steps comprismg:

a. forming a first pattern of lifting material on a substrate,

leaving exposed a portion of the surface of said substrate;

b. depositing a layer of arsenic sulfide over said pattern and theexposed portion of said surface; and,

c. removing said lifting material and the overlying arsenic sulfidewhile leaving arsenic sulfide on the surface of said substrate therebyforming said arsenic sulfide into a second pattern.

3. The method defined in claim 2 wherein said lifting material is aphotoresist.

4. The process defined in claim 2 wherein said lifting material iscalcium fluoride.

5. The method of claim 2, including the additional step of melting saidlayer of arsenic sulfide subsequent to the forming of said arsenicsulfide into said second pattern;

6. In a process for forming masks by photofabrication, the stepscomprising:

depositing a layer of arsenic sulfide over the surface of a substrate;

forming a pattern of masking material on said arsenic sulfide layerleaving exposed portions of said arsenic sulfide layer;

removing the exposed portions of said arsenic sulfide layer;

and

removing the pattern of material overlying the remaining arsenic sulfidelayer thereby to leave a portion of said arsenic sulfide layer on saidsurface in the form of said pattern.

8. The process defined in claim 6 wherein said arsenic sulfide is formedinto said pattern by dissolving the exposed portions of said arsenicsulfide in a basic solution.

8. The method of claim 7 wherein said basic solution is dilute ammonia.

9. The process of claim 6, including the additional step of melting saidlayer of arsenic sulfide after removing said pattern of material;

2. In a process for forming masks that are employed in the manufactureof devices by photofabrication the steps comprising: a. forming a firstpattern of lifting material on a substrate, leaving exposed a portion ofthe surface of said substrate; b. depositing a layer of arsenic sulfideover said pattern and the exposed portion of said surface; and, c.removing said lifting material and the overlying arsenic sulfide whileleaving arsenic sulfide on the surface of said substrate thereby formingsaid arsenic sulfide into a second pattern.
 3. The method defined inclaim 2 wherein said lifting material is a photoresist.
 4. The processdefined in claim 2 wherein said lifting material is calcium fluoride. 5.The method of claim 2, including the additional step of melting saidlayer of arsenic sulfide subsequent to the forming of said arsenicsulfide into said second pattern;
 6. In a process for forming masks byphotofabrication, the steps comprising: depositing a layer of arsenicsulfide over the surface of a substrate; forming a pattern of maskingmaterial on said arsenic sulfide layer leaving exposed portions of saidarsenic sulfide layer; removing the exposed portions of said arsenicsulfide layer; and removing the pattern of material overlying theremaining arsenic sulfide layer thereby to leave a portion of saidarsenic sulfide layer on said surface in the form of said pattern. 8.The process defined in claim 6 wherein said arsenic sulfide is formedinto said pattern by dissolving the exposed portions of said arsenicsulfide in a basic solution.
 8. The method of claim 7 wherein said basicsolution is dilute ammonia.
 9. The process of claim 6, incluDing theadditional step of melting said layer of arsenic sulfide after removingsaid pattern of material;